Autocollimating reflector and application to optical signaling and night advertising



March 19,' 1929. H RETIEN 1,7os,"21s

AUTOCOLLIMATI NG L C-TOR. D APPLIGATI T0 OPTICAL SIGNALING AND NI ADJIERTIS Eiled'Nov. 17, 1924 I direction of incidence, whatever be that di- I Patented Mar. 19 1929.

v UNITED STATES 1 8 PATENT OFFICE.

HENRI ennn'rrnn, or ST.-CLOUD, FRANCE. R S S U AUTOCOLLIMATING REFLECTOR AND APPLICATION TO OPTICAL SI-NALING NIGHT ADVERTISING.

Application filed November 17, 1924, Serial No. 750,420, and in France November 26, 1923, i

As it is known, autocollim'ating reflectors have the property of refiecting light in the rection relatively to the instrument.

An example of a theoretically perfect autocollimating reflector is the Airys triple mirror, which. is obtained by disposing in one support three plane mirrors orthogonally to each other. Any ray of light which impinges onthc three mirrors successively undergoes a change of path which sends it back to its original direction. The triple mirror shows however a fundamental disadvantage: the adjustment of the perpendicularity of the mirrors is very delicate and must be made frequently, especially for transportable instruments. To overcome that difficulty, the triple mirror has been made in the shape of a trirectangle tetrahedroncut out of a block of glass. But such an instrument is difficult of construction, expensive and relatively heavy. Furthermore, the divergency of the emergent beam (or of the beam, whenthe apparatus is not exactly adjusted) is always .very small, which renders it unusable for most of the practical applications here considered.

Another scheme known for a the autocollimating sight F izeau, and which consists of an astronomical lens combined with a small plane mirror placed atits focus orthogonally to its opti-. cal axis. If such a combination is illuminated with a beam of light emanating from a source located'far away, the lens gives an image of it at'the focus, that is to say on the mirrors plane. The mirror sends'the rays back, and those rays, after getting once more through the lens go-back accurately long time is to the source, even if the mirror is not perfectly perpendicular to the optical axis.

The autocollimating sight such as Fizeau conceived'it, is not suitablefor optical signaling, because partlyof its size and partly because of the smallness of its field.

This invention aims to improve this known scheme and comprises I I. Improving the autocollirnating scheme of Fizeau in vorderto render it usable for.

optical signaling.

II. Introducing a divergency ofany given amount in the emerging beam, so as to make the same observable from aregion of chosen size all around the source of'light.

III. Making autocollimating refleetinginvented by surfaces of any size and shape, by assembling simple elements of autocollimating reflectors.

IV. Application of those surfaces to optical signaling in general and to night ad-' I mprooements in the autooollimtz'ng scheme of Fizeau.

The objective system can be hereconsti- 1 tuted by an ordinary type. of astronomical lens, but of focal length reducedas much as possible.

In case of application to secret optical signaling, where the divergency must be at minima, it is important that the objective lens be well corrected of the spherical aberration for utilization on the axis, and of the coma flare for utilization out of the axis. The two components of the lens must be cemented together in order to avoid the loss of light by reflectionon the air surfaces.

It is known that the .optical conditions required can be actually fulfilled with two glasses cemented together keeping thg sys-. tems achromatism by choosing conveniently the dispersive owers of the glasses.

Besides, considering selective action of the J atmosphere through which blue and purple light is quickly absorbed, it is not necessary .in ordinary glass,-to obtain a lens free from sp erical aberration and from-coma for the .yellow radiations and suflieiently achromatic for its .desi ned use.) g

With sllc lens, a oodluxninous'field will readily be obtaine with any lanemjrror placed in the focal plane, rovided, that-the incident pencil does, not j eviatetoo. much from the o tical axis of the system. no

'Butif t e incident pencil; deviates conto conform rigourously to the achromatism condition. Practically it is possible, by ussiderably from the axis the reflected pencil will be notably diminished in its intensity, due principally to a part of the cone of reflected rays intercepted bythe mounting .of the lens.

To get rid of this inconvenience, it is sufficient to change Fizeaus lane mirror into a concave one have a ra ius of curvature equal to the focal len th of the lens. In such conditions the w ole of the incident cone will be reflected through the lens.

However there is still two causes of loss of light for extra axial pencils: thefy areastigmatism and curvature of field o .the 1mage surface. To lessen .this defect the mirror must be composed of a lens a (see Fig. 1) having the shape of a divergent meniscus metallized on its convex side, the concave one being turned towards the lens I). The convex side will have a radius of curvature equal to the focal length of the whole system and its apex will coincide with the focus of thewhole.

The radius of curvature of the concave surface will be determined by the condition of giving to the curvature of the image surface the same value as that of the reflecting surface.

Instead of the catadioptrical disposition, which has just been described, sometimes a disposition simply catoptric can be used with advantage in which the object-lens is replaced by a mirror I) of convenient dimensions. The apparatus shows then the arrangement represented on Fig. 2.

To reduce the diver ency to its minimum, the mirror Z) must be free of spherical aber ration (parabolic mirrorfor Mangin mirror, for instance).

The diameter of the mirror a is determined by the field required for the sight; it will be chosen as small as possible so as not to stop too large a fraction of incident light. of positions which can be occupied by the source 0 is more extended angularly in one direction (horizontal) than in the other (vertical). Thiscircumstance can be used 'to reduce the dimension of the mirror a to what is strictly necessary to cover the Whole extent of the field of the image 0; So the outline of the mirror at, instcadof being circular,..can be rectangular. In some cases, the may even be constituted by singly cylin rical reflecting plates.

Systematic introduction of agioen diver- A gency in the e'nwrg'mg pencil.

' Every time that the purpose is not signaling at great distances, it is necessary to in- Ltroduce systematically in the reflected luminous pencil, a certain divergency in order to It may often happen that the field As the light of the signals will be smaller as the introduced divergency will be greater, this divergency shall be determined for each particular application.

When Airys triple reflector is used, the divergency will be obtained either by leaving a voluntary imperfection in' the plane of the surfaces, or by placing a feebly divergent or convergent lens before each element, or when the trihedral is constituted by three surfaces of a tertrahedron by giving to the fourth surface a shape slightly convex or concave or ,even simply irregular, as it can be directly obtained by molding.

When. using the autocollimating element derived from the .Fizeaus sight, the diver gency is obtained by placing systematically the mirror in front or behind the focal surface of the objective system, by an amount easily determined by calculation or better empirically.

A feeble divergency is obtained when leaving optical aberration, notably spherical aberration in the autocollimating elements, which has besides the advantage of reducing considerably their cost.

As an example of autocolliinating elements where spherical aberration is not especially correct-ed, we can point out: 1. Catoptrical elements previously described but in which the objective-mirrors are affected with spherical aberration we will take for instance simple spherical mirrors the quality of which need not be very good, like those obtained by bulging (glass) or stamping (metal). I

2. A catadioptrical element in which the objective lens 5 and the mirror a are made of one single piece (see Fig. by a lens whose limiting surfaces Z) and a are spheres,

centered in d with their active surfaces, opposite. If 1" is the radius of curvature of the objective surface 6; n, the refraction index of the substance in which the lens has the radius R. The reflecting surface a will be then By selecting a substance with a very high refracting index which theoretically should be equal to 2. but can practically notably differ from this limit (dense lead glasses, strass, etc.) we can bring together the values of R and that of 1' and constitute the autocollimating element by a ,simple spherical ball rendered reflecting on one of its hcmi- I? spheres (Fig. 4).

Autocollinmti'ng reflecting surfaces.

In, order to increase the luminous power of the autocollimatmg reflectors, we can enlarge thelr dimensions. This method prellll) sents however great inconveniences which are as follow.

1. From a certain diameter, the net. cost rises much more rapidly .than surface.

2. The weight of the instrument increases 7 like power 3/2 of the luminosity.

5 3. The absorption of light in lenses rows very rapidly with their thickness, whlch is in proportion to their diameter, from which results a great reduction of luminosity.

4. And again in a conditionofoptical action settled beforehand the length of the instrument isv proportional to its diameter.

The method proposed in conformity with' in another, the two supports being joined by crossbfis of a convenient length to bring the mirrors to focus, a condition that can be obtained by adjusting the divergency to be introduced inthe reflected pencil and by altering the distance-from the lens to the mirror. l

Catadioptric autocollimating surfaces can also be obtained, as shown Fig. 5,'by'fixing on'eachside of a transparent plate e of convenient thickness, convex lenses b and a plac ing them on a sheet of glass,- the objective I 40 lens on one side and the inirroron other,

the latter being metallized on its convex surface. a

y We can proceed in the same way in order to obtain catoptical surfaces, by giving to the two lenses convenient rad'i of curvature and by metallizing the two 0 surfaces (Fig. 6). i

their convex The autocollimating element can be placed on a shop window and to prevent the visibilsoiity of the mirrors from the outside}. such mirrors may be coated with any. opaque covering.

Finally, autocollimating retractingsurfaces io f any dimension and shape can-be obtained by shaping of the'surface of. the

autocollimating element of "any type on transparent sheet (glass, celluloid, cello phan Iii the case of catadioptrical elements, .the go surfacies are'obtained (see Fig. 7) by. shaping p ates of transparent substances so "as L-toI form on each surface a spherical apexwith the radii r-t-R indicatedfabove, and

directly opposite toeach other. I The value T 1"+R, which ensures the concentricity of the;

spheres, is given to the thickness between apexes.

The waved surface formed by the mirror a is rendered reflecting by a suitable metallic deposit.

In the case of catoptric elements, the surface constituting the mirrors a is metallized whole surface can receive a half transparent metallic deposit.

' -Very large autocollimating' surfaces can be constituted by juxtaposition of smaller on a small area,-on the mirror axis b, or the,

ones easier to obtain, as practiced for brick paving or covering of all kinds Fig. 8 shows a disposition of this type, when on'a tray is grouped a great number of systems of obj ective-mirror b a.

Applications.The autocollimating elements derived from Fizeaus sight, previous- 'ly described, those obtained by introducing divergency in Airys trihedral mirror, and the reflecting surfaces obtained by grouping autocollimating elements ofany nature, are

applicable in general to all sorts of optical signaling without its own sourceof light.

From those applications, the following are indicated as descriptive but not limited examples.

Optical tqZegraphy.Elements very small i divergency used alone or by any mechanical appliance, allowing its use in a position having itself no source of light whichcould betray its presence so as to be able secretly to communicate with a lighting hating:

group, operated by hand or provided with post situated very far away for instance an,

agent corresponding over the enemys camp; ships at sea corresponding'between them or with the coast; or an airship in optical communication with a post on the. ground etc....

Optical signaMag.Signals of any sort can be constituted by autocollimatin reflec tors with more or less divergency an which appear luminous to those who are quite near a source of light for exam 1e: i

All. railway signals lighted by sources placed on engines, and' behind which stand those who observe the trackthis eliminates dangers coming from accidental extinctions of the lamps; corner post indicators; dan- "gerousdescent; railroad crossings etc. placed. along the roads in, the country and which lighted by lanterns or'search lights of wagons or cars, shine'very brightly to the conductors eyes and impose themselves upon his attention. v

Indicatorf names of streets, numbers of houses etc. readable: at night by approachmg any small luminous source in front of'the eye; pocket electric ,lamp, flint,even an ordinary match.- X

1 2 Numb placedbeh' d. wagons 01' ships,

train back lanterns, numbers placed in front. oftheengines;1

A Zve7'tising.Advertising sign, placed along roads or railways legible at night Without special light and by simple refiec tion to the eyes of the travelers, of the almost totality of the light coming from I lamps in the cars, can be constituted.

vertising signs legible by those persons Who 7 are near the lights of a caf' for instance.

On a screen of this description a plurality of pictures may be .projected simultaneously from diiferentsources'and angles. These signs although projected on the screen at the same time as those of a different characterdo not become confused one with the other as each separate picture is only visible from the angle of projection.

A'viation.-Autocollimating surfaces can be constituted Which placed horizontally on the ground at intervals Will give at night to aviators flying above them useful indications, such as direction of the wind, with indications such as these it'Would not be necessary for an aerodrome to exhibit lighted signals. If the autocollimating elements are placed at exactly measured distances from each other, the aviator can determine his exact altitude above the ground at night,

by means of any single stadimetric device and so may choose the exact moment for landing.

, of the combination of the two lenses and the other lens having its outer face formed as a reflector.

2. An auto-collimating sight, comprising an objective lens of short focal length and an alined smaller lens having its exterior surface a reflector and smaller than the objective, the radius of curvature of the objective lens being substantially equal to the focal length of the combination of the two lenses and the reflecting surface of the Smallor lens lying substantially in the focal plane of the objective.

3. fin auto-collimating sight, comprising an objective lens of short focal length secured to one face of a sheet of glass and a smaller lens secured to the opposite face of said sheet, the outer faces of both lenses being reflectors and the reflectingouter face of the smaller lens being substantially in the focal plane of the larger lens.

In testimony that I claim the foregoing as my invention, I have signed my name.

HENRI GHRETIEN. 

